#include <stdio.h>
#include <sys/types.h>
int main()
{
printf("pid_t: %zu
", sizeof(pid_t));
printf("uid_t: %zu
", sizeof(uid_t));
printf("gid_t: %zu
", sizeof(gid_t));
}
EDIT: Per popular request (and because, realistically, 99% of the people coming to this question are going to be running x86 or x86_64)...
On an i686 and x86_64 (so, 32-bit and 64-bit) processor running Linux >= 3.0.0, the answer is:
pid_t: 4
uid_t: 4
gid_t: 4
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